EP 0015258 discloses hearing faculty testing apparatus comprising an aural probe for insertion into a subject's external ear canal. The probe includes an electroacoustic transducer for projecting sound into the ear canal and for responding to sound waves returned from the inner ear in response to the projected sound. The transducer is energised by a pulse generator which is said to have a maximum useful frequency of about 50 Hz, because at higher frequencies the echoes overlap with succeeding pulses. Throughout, this prior patent specification refers to extracting echoes by electronic time gating. The need to avoid overlapping and to extract the echoes by time gating represents a disadvantage which the present invention aims to overcome.